The present invention relates to the production of low molecular weight silanol end-stopped diorganopolysiloxanes and more particularly the present invention relates to the production of low molecular weight silanol end-stopped diorganopolysiloxanes from cyclic trisiloxanes with the use of an aprotic solvent acting as a catalyst.
Low molecular weight silanol-stopped diorganopolysiloxanes having an average number of anywhere from three to 10 diorganosiloxy units have many uses including as intermediates for the production of high molecular weight polymers, as additives for the preparation of heat vulcanizable silicone rubber compositions and room temperature vulcanizable silicone rubber compositions and also as additives for the preparation of organic rubbers. In particular, such low molecular weight silanol-stopped diorganopolysiloxanes have found wide use as process aids. Such process aids are utilized as additives to both fumed silica and precipitated silica fillers that are added to mix with diorganopolysiloxane gums in the preparation of heat vulcanizable silicone rubber compositions. Such silanol-stopped materials are added with the filler and gum as the filler is incorporated or mixed into the high molecular weight diorganopolysiloxane gum so as to facilitate the intermixing of the filler and the gum.
The silanol end-stopped diorganopolysiloxanes also improve the final properties of the heat vulcanizable silicone rubber composition in both the cured and uncured state. In the cured state the silanol end-stopped diorganopolysiloxanes impart elasticity. In the uncured state the silanol endstopped diorganopolysiloxanes prevent excessive hardening upon storage.
Another important use of the low molecular weight silanol-stopped diorganopolysiloxanes of the present case is that they may be used as intermediates and condensed so as to form high molecular weight polymers, such as fluorosilicone polymers of high molecular weight, in a manner in which the final viscosity of the polymer is precisely controlled. These fluorosilicone-containing polymers can be, of course, utilized to prepare solvent resistant, fluorosilicone elastomers.
In the past, one method for producing such silanolstopped diorganopolysiloxanes of low molecular weight was to take diorganodichlorosilanes of 99 percent purity or so and hydrolyze them in water in the presence of large amounts of polar solvent. While such diorganodichlorosilanes were added to the water polar solvent mixture there was continuously present, or added at the time of addition of the diorganodichlorosilanes, large amounts of sodium bicarbonate so as to maintain the neutrality of the hydrolysis solution. Both the sodium bicarbonate and the excess amounts of polar solvent were necessary in order to prepare the low molecular weight silanol-stopped diorganopolysiloxanes. If enough polar solvent was not utilized or if enough sodium bicarbonate was not added the silanol groups of the resultant silanol-stopped diorganopolysiloxanes would condense with each other forming high molecular weight silanol-stopped diorganopolysiloxanes which are not considered to be particularly desirable.
This process outlined above is undesirable for various reasons, one of which is the large amount of buffering agents that have to be added to the hydrolysis mixture, thus necessitating very large equipment space. In addition, the salted water phase that is formed after the hydrolysis reaction has to be purified before it can be disposed of. Further, the yield was is lower than expected since the water layer, after the hydrolysis reaction, has to be separated from the polar solvent layer. During this separation step some of the desired end product is lost in the water layer. Accordingly, as one purpose of this invention, it is desired to improve on this process.
One means accomplishing this purpose is disclosed in Omietanski, U.S. Pat. No. 3,309,390. Omietanski discloses the production of low molecular weight silanol-stopped diorganopolysiloxanes from cyclic trisiloxanes through the use of an ion exchange resin. A disadvantage with the Omietanski process is that it uses an ion exchange resin. Such ion exchange resins are unduly expensive. As such they cannot be thrown away after they have spent themselves, and time and effort must be made to regenerate them. The additional time and chemicals necessary to regenerate the ion exchange resin, of course, adds to the expense of the process. In addition, such ion exchange resins may have a certain amount of residual acidity on the resin which has to be washed off before it can be utilized in the Omietanski process. Otherwise, the low molecular weight silanol-stopped diorganopolysiloxanes that are formed from the process may contain an excessive amount of acidity which will result in the condensation of the silanol groups of the desired product.
It has also been found that acid-activated carbon black will not operate in such a process. When used, the desired low molecular weight diorganopolysiloxanes are not obtained.
Another means for accomplishing the purposes of this invention is taught by Razzano in U.S. Pat. No. 3,853,932. Therein, cyclic triorganopolysiloxanes are catalyzed by acidactivated hydroaluminum silicate clay in the presence of water and a polar organic solvent. This process, however, suffers from the some of the same problems stated above. Namely, the ring-opening hydrolysis is a multi-step process that requires a number of pieces of equipment. Further, the acid activated clay must be separated from the resultant product in order to avoid stability problems. This separation process is costly, requires additional equipment, and results in product loss and decreased efficiency.
Accordingly, it is one object of the present invention to provide an inexpensive and efficient catalyst for the production of low molecular weight silanol-stopped diorganopolysiloxanes from cyclic trisiloxanes.
It is an additional object of the present invention to provide a process for producing low molecular weight silanolstopped diorganopolysiloxanes from cyclic trisiloxanes so as to obtain such low molecular weight silanol-stopped diorganopolysiloxanes in high yield.
It is an additional object of the present invention to provide a process for producing low molecular weight silanolstopped diorganopolysiloxanes which can be used as intermediates to produce high molecular weight fluorosilicon polymers.
These and other objects of the present invention are accomplished by means of the disclosure set forth below.